The present invention relates to a carbonaceous composite material superior in strength and immune to warpage, a process for efficient production of said carbonaceous composite material, a fuel cell separator made of said carbonaceous composite material, and a polymer electrolyte fuel cell superior in vibration and shock resistance.
A fuel cell system, particularly that of solid high polymer type, consists of tens or hundreds of unit cells"" which are stacked to form the battery module. As shown in FIG. 1, each unit cell is made up of one electrolytic membrane of solid high polymer 2, two gas diffusion electrodes of carbon paper 3, and two separators 1, 1, each having ribs 1a which form grooves 4 for gas (such as hydrogen and oxygen) to be supplied and discharged.
The fact that each unit cell is low in output voltage makes it necessary to stack tens or hundreds of unit cells in order to construct a fuel cell system with a practical capacity of the order of 100 kW. This has aroused a demand for efficient mass-production of fuel cell separators.
Unfortunately, the conventional fuel cell separator, which is composed mainly of a thermosetting resin and graphite, lacks fluidity on account of a large amount of graphite incorporated to impart electrical conductivity. Therefore, it presents difficulties in its injection molding and transfer molding. It is usually produced by the compression molding process which consists of placing the compound in a mold and pressing it at 150-160xc2x0 C. and 14.7-29.4 MPa for 5-10 minutes. Compression molding, which takes a long time, is inefficient for mass production.
The fuel cell separator 1 mentioned above has a unique structure with a number of grooves 4 for gas supply and discharge which are formed on one or both sides of a thin plate, as shown in FIGS. 2(A) and 2(B). The thin bottom of the groove causes the fuel cell separator to warp or crack easily.
The present invention was completed in view of the foregoing. Accordingly, it is an object of the present invention to provide a carbonaceous composite material superior in strength and immune to warpage, a process for efficient production of said carbonaceous composite material, a fuel cell separator made of said carbonaceous composite material, and a polymer electrolyte fuel cell superior in vibration and shock resistance.
In order to achieve the above-mentioned object, the present inventor carried out a series of researches, which led to the finding that a fuel cell separator is obtained from a carbonaceous composite material composed mainly of graphite, a thermosetting resin, and a fibrous base material by molding in such a way that said fibrous base material is oriented at 60-120 degrees with respect to the thickness direction of thick parts. The thus oriented fibrous substrate effectively functions as a reinforcement, thereby preventing warpage and adding strength to thin parts (or weak parts). The resulting fuel cell separator has good strength.
Moreover, the present inventors carried out extensive studies on the process for efficiently producing the carbonaceous composite material. As the result, it was found that the fibrous base material imparts an adequate degree of fluidity to a mixture composed mainly of graphite and a thermosetting resin, thereby rendering the mixture suitable for injection molding and transfer molding.
To be concrete, a mixture composed of 100 pbw of graphite, 15-45 pbw of thermosetting resin, and 3-35 pbw of fibrous base material is suitable for injection molding and transfer molding on account of its adequate fluidity due to each component. The mixture does not stick to the screw of injection or extrusion molding machines, nor does it suffer slipping between the screw and the cylinder (due to compression and sticking). Thus it is possible to eliminate difficulties which have previously been encountered in injection molding and transfer molding. Molding is carried out in such a way that the fibrous base material is oriented at 60-120 degrees with respect to the thickness direction of thick parts. Therefore, the fibrous base material protects the molded product from warpage and reinforces thin parts (or weak parts). The above-mentioned mixture and molding method permit the efficient mass-production of strong warp-free fuel cell separators. These separators are suitable for the polymer electrolyte fuel cell which need good vibration and shock resistance.
The present invention provides a carbonaceous composite material, a process for production thereof, a fuel cell separator, and a polymer electrolyte fuel cell, which are defined in the following.
(1) A carbonaceous composite material molded from a carbonaceous composite compound composed mainly of graphite, a thermosetting resin, and a fibrous base material, wherein molding is carried out in such a way that said fibrous base material is oriented in said carbonaceous composite material.
(2) A carbonaceous composite material as defined in (1) above, wherein the fibrous base material is oriented at 60-120 degrees with respect to the thickness direction of the thick parts of the molded product.
(3) A carbonaceous composite material molded from a carbonaceous composite compound composed mainly of graphite, a thermosetting resin, and a fibrous base material, wherein said graphite, thermosetting resin, and fibrous base material account for 100 pbw, 15-45 pbw, and 3-35 pbw, respectively, in the total amount.
(4) A carbonaceous composite material molded from a carbonaceous composite compound composed mainly of graphite, a thermosetting resin, and a fibrous base material as defined in (1) or (2) above, wherein said graphite, thermosetting resin, and fibrous base material account for 100 pbw, 15-45 pbw, and 3-35 pbw, respectively, in the total amount.
(5) A process for producing a carbonaceous composite material by transfer molding from a mixture composed of 100 pbw of graphite, 15-45 pbw of thermosetting resin, and 3-35 pbw of fibrous base material.
(6) A process for producing a carbonaceous composite material by injection molding from a mixture composed of 100 pbw of graphite, 15-45 pbw of thermosetting resin, and 3-35 pbw of fibrous base material.
(7) A process for producing a carbonaceous composite material as defined in (5) or (6) above, wherein the fibrous base material is oriented at 60-120 degrees with respect to the thickness direction of the thick parts of the molded product.
(8) A fuel cell separator having grooves for gas supply and discharge on one side or both sides thereof which is formed from the carbon composite material defined in any of (1) to (4) above, said fuel cell separator being characterized by a flexural strength of 40-100 MPa, a flexural modulus of 20-60 GPa, and an amount of warpage no larger than 0.5 mm, which are measured according to JIS K6911.
(9) A polymer electrolyte fuel cell which is made up of a number of unit cells, each cell consisting of a pair of electrodes holding an electrolytic membrane of solid high polymer between them and a pair of separators holding said electrodes between them, said separator forming passages for gas supply and discharge, wherein the separator is entirely or partly the fuel cell separator defined in (8) above.